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You can just back up your 3G and when the 4S arrives, plug it in, then you can choose to restore from that backup.
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Right I have plans to do so but when I restore from that backup it won't have iOS5 on it so will I then have to reinstall iOS5 on it?
It will just restore your backup to your new shiny iPhone 4S with iOS 5. The iOS/firmware/baseband whatever won't be downgraded in the process.